1. Field of the Invention
The present invention relates to a fuel injection system of an engine, in particular, to a direct fuel injection engine in which a fuel injector is so arranged that a nozzle of the injector is directed into a combustion chamber to inject a fuel at a later stage of a compression stroke in at least a low load and low engine speed condition of the engine.
2. Related Art
As disclosed in Japanese unexamined patent publication No. 6-81651 published in 1994, a direct injection engine has been known that is provided an ignition plug, injector for directly injecting a fuel into a combustion chamber to facilitate a laminar combustion so as to improve a fuel consumption efficiency at a later stage of a compression stroke in at least the low load and low engine speed condition of the engine. In this type of engine, the fuel is injected into the chamber while suppressing a diffusion of the fuel so as to facilitate the lamination of the intake gas in the chamber. In this case, it is necessary to suppress the diffusion of the fuel but facilitate an atomization of the fuel and locate a combustible intake gas mixture of an appropriate air fuel ratio around the ignition plug.
For this purpose, the direct injection engine disclosed in the above Japanese publication, a piston is formed with a recessed and curved surface on its top surface and a raised portion continued from the curved surface. The injection is arranged so as to inject the fuel toward the recessed and curved surface when the piston attains to substantially a top dead center. The ignition plug is arranged against the raised portion so that the fuel injected from the injector hits the curved surface of the piston to diffuse to be mixed with the air and stay around the ignition plug.
Japanese Un-examined Patent publication No. 8-165975 also discloses a direct fuel injection engine structure in which a recess or cavity is provided on the top surface of the piston wherein the injected fuel passes through the ignition gap.
As aforementioned, in such conventional direct injection engine, the fuel from the injector is directed to hit a wall of the combustion chamber (for example, the top surface of the piston). In this way, a part of the fuel hit the wall stick to a surface of the wall and is left thereon without diffusion. This fuel tends to be emitted without a complete combustion. As a result, a hydrocarbon mixture in an exhaust gas is increased to deteriorate not only an emission performance but also the fuel consumption efficiency.